A Cathode Ray Tube (CRT) display generally provides the best brightness, highest contrast, best color quality and largest viewing angle of prior art displays. CRT displays typically use a layer of phosphor which is deposited on a thin glass faceplate. These CRTs generate a picture by using one to three electron beams which generate high energy electrons that are scanned across the phosphor in a raster pattern. The phosphor converts the electron energy into visible light so as to form the desired picture. However, prior art CRT displays are large and bulky due to the large vacuum bottles that enclose the cathode and extend from the cathode to the faceplate of the display. Therefore, typically, other types of display technologies such as active matrix liquid crystal display, plasma display and electroluminiscent display technologies have been used in the past to form thin displays.
Recently, a thin flat panel display (FPD) has been developed which uses the same process for generating pictures as is used in CRT devices. These flat panel displays use a backplate including a matrix structure of rows and columns of electrodes. One such flat panel display is described in U.S. Pat. No. 5,541,473 which is incorporated herein by reference. Typically, the backplate is formed by depositing a cathode structure (electron emitting) on a glass plate. The cathode structure includes emitters that generate high energy electrons. The backplate typically has an active area within which the cathode structure is deposited. Typically, the active area does not cover the entire surface of the glass plate, leaving a thin strip around the edges of the glass plate. Traces extend through the thin strip to allow for connectivity to the active area. These traces are typically covered by a dielectric film as they extend across the thin strip so as to prevent shorting.
Prior art flat panel displays include a thin glass faceplate having one or more layers of phosphor deposited over the interior surface thereof. The faceplate is typically separated from the backplate by about 1 millimeter. The faceplate includes an active area within which the layer (or layers) of phosphor is deposited and a thin strip that does not contain phosphor. The thin strip extends from the active area to the edges of the glass plate. The faceplate is attached to the backplate using a glass sealing structure. This sealing structure is formed by melting a glass frit in a high temperature heating step. This forms an enclosure which is evacuated so as to produce a vacuum between the active area of the backplate and the active area of the faceplate. Individual regions of the cathode are selectively activated to generate high energy electrons which strike the phosphor so as to generate a display within the active area of the faceplate. These flat panel displays have all of the advantages of conventional CRTs but are much thinner.
In another prior art flat panel display design, a ceramic frame is placed between the glass faceplate and the backplate. Glass frit is placed on each side of the ceramic frame and the flat panel display assembly is heated. The glass frit is heated so as to form a seal between the ceramic frame and the backplate and a corresponding seal between the ceramic frame and the faceplate.
In prior art fabrication processes, a hollow evacuation tube is placed such that it extends across the thin strip of the backplate. Typically a glass or copper tube is used as the evacuation tube (also referred to as a pump port). A thin layer of glass frit is then deposited around the backplate such that the glass frit surrounds the active area of the backplate. The enclosure is only interrupted by the evacuation tube which extends across the layer of glass frit.
The faceplate is then placed over the glass frit on the backplate such that the active area of the faceplate is aligned with the active area of the backplate. The resulting flat panel display assembly is then placed in an oven where a high temperature process step is performed so as to melt the frit. The glass frit forms a seal between the faceplate and the backplate as it melts, forming an enclosure into which the evacuation tube extends. Typically, a temperature of at least 400 degrees centigrade is required to melt the glass frit.
The flat panel display assembly is then removed from the oven and a vacuum hose is attached to the evacuation tube. Any gas within the enclosure is then removed through the evacuation tube. The evacuation tube is then sealed off and the vacuum hose is removed. The resulting display assembly has a sealed enclosure which has a vacuum formed therein.
The bonding process is time consuming and expensive due to the numerous fabrication steps. In addition, the high temperatures required during the sealing process damages the emitters so as to degrade the cathode. Also, the setup and down cycle during the sealing process induces stress to the faceplate and the backplate. Moreover, the high temperatures cause the structures on the surfaces of the display assembly to outgass (Typically, polymer present on the surfaces of the faceplate and the backplate is outgassed). This outgassing results in contaminate species absorbed by the active area of the backplate or faceplate. The outgassed contamination of degrade or oxidize the emitter surface causing electron emissions to be temporally unstable and in general, reduced. In addition, ions formed through the collision of electrons with gas molecules can be accelerated into the emitter tips and may therefore degrade their emission. Plasma formed in the same manner can short emitter tips to the overlying gate and can cause arcing at high field regions in the display. Thus, outgassing interferes with the operation of the cathode, resulting in reduced image quality.
Outgassing is reduced in prior art flat panel display by the use of materials that have a low outgassing rate and that have a low vapor pressure. Thus, only metals, glasses, ceramics, and select specially processed polymers are typically used within flat panel displays. These materials are typically processed by baking (at several hundred degrees centigrade) and electronically or otherwise scrubbing in order to remove adhered molecules. However, only some of the outgassing may be eliminated by such processes. Thus, the materials, and in particular, the polymer surfaces outgass during the high temperature steps of prior art processes, producing harmful O.sub.2, H.sub.2 O, CO, and CO.sub.2. Typically, a getter is used to minimize damage resulting from outgassing. The getter absorbs some of the chemicals released by outgassing. However, getter only absorbs certain outgassing moleculars, allowing the remainder of the damaging moleculars to fall onto the active surfaces of the flat panel display.
Alternate prior art heating methods for forming a seal between the faceplate and the backplate include the use of lasers which are focused on the glass frit. Typically, such methods heat the glass frit to temperatures of more than 600 degrees centigrade. However, since the heat is localized, the damage such as oxidation to the active areas is reduced. Damage resulting from oxidation is typically reduced by performing the heating process in an inert gas environment such as nitrogen. However, in order to prevent the glass of the faceplate and the backplate form cracking or breaking from the sudden temperature increase and a large temperature difference between the components, the display assembly must be heated in an oven to the glass transition temperature which is typically 300 to 325 degrees centigrade. This high oven temperature causes oxidation which results in cathode degradation. Moreover, the 325 degree temperature stresses the surfaces of the faceplate and the backplate and causes a significant amount of outgassing.
In an attempt to solve the inherent in prior art sealing process, prior art display assemblies employing pump ports and/or evacuation tubes, have attempted to heat the display assembly in a vacuum. However, glass frit is not stable at high temperatures in a vacuum, resulting in disassociation of the glass structure (2PbO.fwdarw.2Pb+O.sub.2). The resulting lead and oxygen causes oxidation and contamination. Moreover, the high temperature of the sealing process results in stress to the faceplate and to the backplate and cathodic degradation and outgassing. Though the use of inert gasses such as nitrogen eliminates the problems associated with oxidation, these prior art processes still damage the active surfaces due to stress and outgassing.
With an evacuation scheme which includes an evacuation tube, the thickness of the display assembly is increased by the length of the evacuation tube. This limits the minimum thickness of the display assembly.
Flat panel display fabrication processes are expensive and the manufacturing process is time consuming due in large part to the number of complex steps required in the bonding process. Moreover, prior art bonding processes are performed at high temperatures, resulting in outgassing and heat generated defects. This decreases yield and increases overall manufacturing cost. In addition, the numerous process steps take up a long process time so as to cause low throughput rates.
Thus, a need exists for a flat panel display and a method for bonding a flat panel display which is relatively inexpensive and easy to manufacture. A further need exists for a flat panel display and a method for forming a flat panel display which does not damage the active areas during the bonding process. In particular, a need exists for a flat panel display and a method for forming a flat panel display which minimizes outgassing and thermal stress. A further need exists for a flat panel display and a method for forming a flat panel display which minimizes fab process time and which reduces manufacturing cost. Moreover, a flat panel display and a method for forming a flat panel display is needed that will increase yield and throughput of manufacturing. The present invention meets the above needs.